Add ResetToSequence to ConsumerManager that updates NextSequence,
clears AckProcessor state via new ClearAll(), and zeroes PendingBytes.
Add AckProcessor.SetAckFloor() that prunes pending entries below the
new floor. Go reference: consumer.go:4241 processResetReq.
Heartbeat frames now include Nats-Pending-Messages and Nats-Pending-Bytes
headers populated from the ConsumerHandle. Flow control frames increment
FlowControlPendingCount; AcknowledgeFlowControl() decrements it. IsFlowControlStalled
returns true when pending count reaches MaxFlowControlPending (2).
Go reference: consumer.go:5222 (sendIdleHeartbeat), consumer.go:5495 (sendFlowControl).
Adds PauseUntilUtc to ConsumerHandle, a new Pause(DateTime) overload,
Resume, IsPaused, and GetPauseUntil to ConsumerManager. A System.Threading.Timer
fires when the deadline passes and calls AutoResume, raising OnAutoResumed so
tests can synchronise via SemaphoreSlim instead of Task.Delay. ConsumerManager
now implements IDisposable to clean up outstanding timers. Timer is also
cancelled on explicit Resume and Delete.
Go reference: consumer.go (pauseConsumer / resumeConsumer / isPaused).
